1. Field of the Invention
The present invention relates to navigation instruments and more specifically to marine speed logs.
2. Description of the Prior Art
Known types of electro-magnetic underwater logs for marine vessels utilize an on-board indicator/transmitter unit which provides an energizing current through a connecting cable to a submerged sensing unit. The sensing unit contains a coil energized by this current so as to establish a magnetic field in the surrounding water. Water flow through this magnetic field as a result of the vessel's motion induces a voltage gradient in the water. The voltage is detected by a pair of metallic buttons in the sensor, and applied to the transmitter by means of a second pair of cables. Such logs must occasionally be calibrated at the zero speed point, which in the past has involved a complicated procedure. One prior art zero point calibration technique involved the establishment of zero water flow past the sensor. This must be done at dockside or at anchor, since establishment of zero speed at sea is difficult and may disrupt other operations. Zero water flow dockside or at anchor may be obtained either by waiting for slack tide or by shrouding the sensor. With either approach, there is a risk of encountering in-water voltages produced by electrical equipment on shore, and therefore incorrectly estimating the zero speed error.
Another technique involves the deletion of the sensor output. This is done either by artificially forcing the sensor signal to zero by short circuiting the sensor output, or by replacing the sensor signal with an artificial signal. This method permits correction of errors originating within the electronic portion of the indicator/transmitter but does not permit compensation for erroneous indications produced by pick-up in the cables connecting the indicator/transmitter to the sensor or for errors associated with the in-water voltages produced by the ship's own electrical equipment.